Mysterious power-amp hum with solid-state guitar amp

[HarpCat]

Hi,

I got a Behringer GMX 212 guitar amp in need of a repair and need some help.
There is a low-level 50Hz Buzz on the right speaker which is constant from moment you turn on power. It's pretty annoying.
On the left speaker there is a very slight 100 Hz Buzz (raspy). It's only hearable when I put my ear directly in front of the speaker.

The hum on both channels gets louder during first 3-4 seconds after amp is turned on. It seems to get louder during first 30 seconds. Especially the left side gets noticeably louder.

If I change of master volume or any of the preamp gain/volume settings, the hum stays the same. So I guess its either a grounding issue (but unlikely since it's a industrial built amp), a problem with the power amp board or some faulty parts in the power supply.

Checked the components and traces of the power-amp/power supply-board visually: Everything looks fine.
Wiggling on connectors, cables, knocking on boards with screwdriver also resulted in no change. Grounding of the transformer and all boards seems OK.

So I continued with more systematic testing.

Maybe 4 years ago, I had to change the input board, because the plastic socket broke. Because the hum seemed to appear a few moths after the change, I attributed it to that. So I disconnected it. No change.

To make sure the fault is in the power-amp, I plugged in two guitar cables in slave in left and right. This is meant as a sort of "line in". It disconnects the preamp and sends the signal from the cables directly to the power-amp.
Now the left side 100Hz buzzing got very much louder. Maybe twice as much as the right side hum. The right side remained unchanged.

To make sure, there wasn't any hum coming from the cables, I unplugged the slave inputs. Then I disconnected preamp board from poweramp (pulled the jumper cables). The effect was the same: the left side 100Hz hum increases significantly.

Also, when I plug in cables into the external speaker jacks, the hum on the speakers disappears. However a very low level hum remains which is transferred onto the chassis. It seams to originate from the power-amp, since its less loud at the transformer.

So i thought, maybe the power-amp is drawing to much current and is loading the power supply. To check, I left the amp on for a half hour, to see if the power-amp gets unusually hot. Nada. It stays pretty much at room temperature.

This symptom, when I disconnect the preamp, gets me a bit puzzled. I thought the hum is coming from the power amp. Why does it increase, when I disconnect the preamp?

Maybe somebody could kindly take a look at the schematic. Do you have any pointers to the next steps I should undertake?

Cheers Thomas


Here's the schematic: http://http://www.electronica-pt.com...down/id,16011/

[nickb]

I wonder if the slave input jack ground is open. Try this. Plug the guitar lead back into the right slave input. Short the ring to the sleeve. Does the hum reduce significantly?

Another thought. If C11 were shorted the low frequency gain would be higher making any hum on the input a little louder, but not enough to account for what you hear, I suspect.

It would be worthwhile to measure and compare the DC volts on the speaker outputs with no signal.

[HarpCat]

Next steps

Hi and thanks for the reply!
What a busy week. Now I found some more time to look at the amp again.

This time I had the amp in a different spot, so I could reach the back better. And now I can correct, that the very low level buzz is indeed from the transformer and not the pa board. So this might probably be normal and is acceptable for me. The buzz coming out of the speakers is not.

I wonder if the slave input jack ground is open. Try this. Plug the guitar lead back into the right slave input. Short the ring to the sleeve. Does the hum reduce significantly?

Shorting the right slave input has no effect. But shorting the left input to ground makes the left side buzz disappear completely.

I also connected the ground shields of both slaves to ground. No effect.

Another thought. If C11 were shorted the low frequency gain would be higher making any hum on the input a little louder, but not enough to account for what you hear, I suspect.

Yea, probably. However, the Sound of the amp is normal and low frequencies are not louder than they should be.

It would be worthwhile to measure and compare the DC volts on the speaker outputs with no signal.

D.C. on Speaker outs is negligible:
Left: 3.6 mV
Right: 3.4 mV

Another thing I discovered while looking at the p.a. board closely: C3 is soldered in crocked and has direct contact to IC1, so it gets heated quite well...

Any further ideas?

[nickb]

Shorting the right slave input has no effect.

The way I'm taking the problem description is that the left channel is good and the right is bad.

When you plug the lead in to the slave jack, I'd expect a general increase in noise on that channel. When you short the cable tip to ring that added noise should go away. if that is not happening then there is something wrong with the ground on that slave input. Also the added noise when open would be much louder on that channel as the cable screen would not be connected. If there was no added noise then there is a problem with the signal side. There's clearly something screwy here.

It is possible to unplug the the power amps by unplugging the cable from X17 on the AUX board to X1 on the power amp board? What effect does that have? If hum/noise is then the same n both channels I think that narrows the problem down to the area of the slave jack and X17/X1 connection.

[1ampman]

I'll take a guess.... Driver transistor shorted (pulling current that gets into the outputs) Or possible open resistor on input impedance reference (goes to grd from signal wire) Now I'm curious?

[J M Fahey]

No visible/reachable driver transistors in an LM3886 chipamp, please check schematic first

Back to amp:
Donīt think C3 is the problem, it does not get *that* hot anyway.

Normal DC at IC2 out and it reproducing clean music through its speaker when driving it through its slave in, which is a direct feed (test that please) strongly hints at a "good" chipamp.

Sounds like a grounding problem ... but this is a PCB.

Just for kicks, take a sharp well illuminated picture from PCB bottom, Iīd love to see the ground track joining all "A" labelled ends of:
C62 - R1 - IC2(pin 7) - C4 - C3 - R10 - R23 - X24 - X9 ... all the way to "ground zero" at the copper area joining C5-C6 , main filter caps.

Tracks can not get loose like a cable but can crack when PCB flexes (vibration, transport here and there) and such cracks are almost invisible (but can be tested).

Edit the picture on Photoshop or something and add a thin yellow or bright green line over it, so we follow the path.

Not a magic bullet by any means, but thatīs what Iīd check if I actually had it on my bench.

Another possibility, and remember I *hate* shotgunning, is to replace IC2 with a fresh one.
Carefully of course, clean solder pads well before pulling it, pamper the PCB (parts can be replaced, PCB can not).

IC2 might have had its input abused, something improper (from speaker or headphone out to static spark coming from a pedalboard supply) *can* be plugged straight into Aux In and there is no protection at all , the puny 1k R41 resistor amounts to nothing at all.

Well designed Peavey and Crate always use a larger value resistor (10k - 15k) in such places, combined with clamping diodes, they are certain that at least some of their amps can be abused by users, even unknowingly, and semi-damaged transistors sometimes become noisy, sometimes even buzzy.

But leave replacement as a last option, in any case order a fresh LM3886, worst case you have it handy and itīs only 5 or 6 bucks. Remember to order proper mica and grease or a SilPad.